But the OS didn't impress me to much, they didn't came up with something new or exciting, just ordinary micro kernel.

First, plain old microkernel? Show me a viable microkernel operating system that really functions like a microkernel OS should--with all device drivers in userspace and only the VM service in the kernel. Sure, MINIX, L4, Mach, Plan9, and others are headed down that path, but none have been successful. If Singularity really was a true microkernel operating system, that would be impressive.

But it isn't. Although Singularity has a really small surface area of trusted code, lots of managed code lives in kernelspace to avoid the overhead of IPC and context switches. Singularity, believe it or not, is closer is design to a mainframe operating system like IBM's VM. Isolated processes are nothing new in general, but they are revolutionary on "new-school" architectures like x86, PPC, and SPARC. The classic mainframes in the 1970s and 80s all had isolated processes, although they were at least partially isolated in hardware. The idea of a strictly software isolated process model might be uncharted territory (not sure).

What whould realy be interesting is if they did an OS not in an imperative language, but in functional, e.g. by incoporating MS other R&D project like F#.

I'm sure that's possible in theory, but it wouldn't be easy, and it certainly wouldn't be pretty. Operating systems are all about side-effects, and functional programming (strictly speaking) doesn't allow them.

C is the language of choice for systems programmers everywhere. The C language is the greatest triumph of computer science. It's the wheel, it's fire, it's sliced bread. As they say, choose the right tool for the right job. But C will continue to be right for many jobs well after the newer languages fall out of favor.